In an engine valve operating mechanism, it is necessary to bias a valve in a valve closing direction with a load generated by a valve spring in order to counteract the inertial force of the valve, which increases in proportion to the square of the rotational speed of the engine. In order to prevent the valve operating mechanism from being damaged due to improper movement of the valve when the rotational speed of the engine exceeds an allowed rotational speed due to a downshift error in a manual transmission, etc., a load that exceeds the valve spring load required for achieving the allowed rotational speed is needed. If the load of the valve spring or the strength of the valve operating mechanism is increased in order to prevent damage to the valve operating mechanism, there is the problem that the weight and the cost will increase.
An engine valve operating system described in Japanese Patent Application Laid-open No. 8-232693 reduces the inertial force of the valve to thereby prevent the occurrence of improper valve movement by increasing the valve opening angle without changing the amount of valve lift.
However, in the above-mentioned conventional arrangement, when the rotational speed of the engine increases and improper movement of the valve is about to occur, in order to decrease the inertial force of the valve, the valve opening angle is increased without changing the amount of valve lift, and although the original desire was to decrease the rotational speed of the engine, since the intake air volume increases, the rotational speed of the engine increases, and there is a possibility that improper movement of the valve might be promoted. Furthermore, when the valve opening angle is increased, since the effectiveness of engine braking deteriorates, there is a possibility of an effective braking effect not being obtained and improper movement of the valve not being suppressed.